Once you know where you want your cord to come out, mark it on the outside of the box. Be sure that the cord comes out low enough that the box can close, but high enough that even when the hole is full-sized, it's a cardboard thickness from the bottom.
Find the center point of the future hole. Pierce it, from the outside in with the awl. (That way, the ragged edges of tin will be on the inside.) Then use the needle-nosed pliers or similar to force the hole larger (again, outside in). Turn it as you go so that you get a round hole, and stop when you can just force the end of the power cord through the hole from the outside in.
The inside of the hole will be a mess. Use pliers to pinch the torn edges of metal flat. Try to pad them so that they don't mark the outside of the tin too much.